As integrated circuit (IC) technology advances, integrated circuit minimum feature geometries and surface cell sizes become smaller. In addition, as conductive region separation becomes smaller over time, capacitive coupling increases between conductive regions. Capacitive coupling is phenomenon wherein undesirable capacitance results between adjacent conductors in an IC circuit. Capacitive coupling usually causes a phenomenon known as cross-talk and induces large conductive region RC (resistive-capacitive) time delays in IC conductors. In order to reduce the effects described above, several isolation methods were either implemented or researched.
One method used to reduce the capacitive coupling, and therefore cross-talk and RC time delays, is to insulate conductive lines and regions with a low permittivity dielectric. An example of a known low permittivity dielectric is boro-nitride. Instead of using dielectrics such as nitride, silicon dioxide (SiO.sub.2), tetra-ethyl-ortho-silicate (TEOS) based oxides, boro-phosphate-silicate-glass (BPSG), or phosphate-silicate-glass (PSG) which have a dielectric permittivity constant on the order of three to four, dielectrics with a permittivity less than three are used. It is known that isolation via low permittivity constant dielectrics reduces capacitance coupling between adjacent conductors. One disadvantage is that this method of capacitive coupling reduction is only able to reduce the capacitive coupling by a maximum factor of three. Greater reduction in the capacitive coupling is required for many applications, and the use of low permittivity dielectrics requires extensive and expensive research of new materials, processes, and/or equipment.
Another method used to reduce capacitive coupling is to replace conventional conductive lines and/or regions with super-conductive lines and/or regions. Although super-conductive interconnects will reduce capacitive coupling effects, super-conductors are expensive to research and can only operate at low temperatures. Therefore, the use of super-conductors in an integrated circuit product may limit product applications or increase maintenance costs and/or operational costs. In addition, super-conductors tend to be difficult to work with chemically and mechanically, unlike other conductive materials such as aluminum, polysilicon, or copper. Super-conductors are not easily compatible with current integrated circuit processing.
Another way to isolate conductive regions is to use an air bridge or coaxial cable approach. A conductive region which has an underlying dielectric layer is over-etched, usually in an isotropic manner, to remove portions of the dielectric layer and leave the conductive layer suspended in the air. The conductive region is then insulated with a shielding dielectric, and the shielding dielectric is covered with a shielding conductive region. The shielding dielectric and the shielding conductive region completely surround the conductive region. This completed structure is very similar to a known shielded coaxial cable structure which is used for long distance signal transmission. All known processes that are used to make a coaxial semiconductor device are extremely complex, are unproven for most integrated circuit technology, and may result in low circuit yield when used for high-volume production.